The present invention relates to a digital communication system; and more particularly, to a digital communication system which has a transmission speed greater than or equal to several bits per second, and which is suitable for performing high-speed transmission between terminals through an exchange, with a telephone line as a transmission medium.
In a digital communication system employing a telephone line, noise having a large amplitude may be mixed in a digital signal over a length of several bits, so that a burst error occurs in some cases. Several conditions may cause such errors; for example, where an analog line and a digital line are provided in the same telephone cable, the linking pulse or the dial pulse of the analog line, generally having an amplitude in the range of several tens of V to 100 V, leaks into the digital line. Similarly, a high-voltage noise on a power source line adjacent to a telephone cable may leak into the digital line of the telephone cable. In such cases, the amplitude of the burst noise is often much more than that of the input signal (in the range of several hundreds of mV to several V) in the digital line, and the frequency or band width of the signal is approximately equal to that of the noise. Therefore, the burst noise cannot be completely removed through a frequency filter often used for the removal of such noise.
Viterbi decoding is one method previously used to correct errors like those described above. An example in which Viterbi decoding has been applied to a partial response class 4 code (hereinafter, "a PR4 code") is disclosed in JP-A-2-67851. According to this technology, the correlation between symbols which are 2 bits away from each other is added by a (1-D.sup.2) coder, and even 2 bit-continuous errors can be corrected by the Viterbi decoding.
However, in the environment of a private telephone line, impulse noise having a width of about 1 .mu.s may occur in some cases. This corresponds to a continuous error of 4 bits when performing the 4 Mbps-transmission. Thus, the problem with respect to the impulse noise cannot be solved perfectly by the above-mentioned prior art technology. With respect to this problem, a continuous error of n bits can be corrected by a coder which performs an encode according to the expression "1-D.sup.n " (hereinafter, "n-th order (1-D) coder"). However, since a (1-D.sup.2) code has already been widely used, it is not economical to convert all of the existing transmitters and receivers. Moreover, long impulse noise having a width of about 1 .mu.s occurs only in very limited situations where a line is in a poor noise environment, so that the (1-D.sup.2) code may cope with impulse noise sufficiently in some cases. Thus, the n-th order (1-D) coder where n.gtoreq.3 is not necessarily required. Therefore, it is ideal that the function corresponding to the (1-D.sup.2) coder and the function corresponding to the n-th order (1-D) coder (where n.gtoreq.3) is selectively used according to the noise condition of the telephone line. To this end, on the transmission side a sender using current technology could select a coding system required for a user according to the state of the line. However, since the decoding system in the receiver must correspond to the coding system in the transmitter, the receiver would have to confirm the coding system every communication by communicating with the sender. Such a handshaking requirement is undesirable for practical use.